At least, while the OS might very well turn out to be non-upgradeable, there is some talk about app compatibility...

(latest iOS on iPhone 3GS is not quite so rosy as many make it out to be - features missing, availability of Siri on pre-4S devices even purposefully removed from appstore, plus don't forget it's a device still actively promoted and pushed on consumers; maybe we should count also from the ending times of large-scale sales)